The present invention relates to a method of obtaining polycrystalline silicon, and more particularly to a method of separating polycrystalline silicon from the substrate on which it is grown, and a workpiece useful in the practice of the method.
Sheets of polycrystalline silicon are used for many applications, one of the applications being in solar cells or photovoltaics. A major factor limiting the use of such sheets in solar cells is the high cost of obtaining a sheet separated from the substrate upon which it is formed. For example, formation of a sheet directly on an expensive highly polished silicon wafer substrate makes it impossible to separate the sheet from the substrate. At the very least, the substrate must be cleaned and repolished after each use.
In an attempt to ameloriate this problem, the wafer has been provided with an oxide, nitride or oxynitride coating and then the sheet formed directly on top of the coating. This has not proven satisfactory as the sheet adheres tightly to the coating and it is almost impossible to remove the sheet from the coating without damaging both the sheet and the coating.
It is known that germanium grown on a thick layer of carbon over a substrate is easily separable from the quartz substrate. In an attempt to ameloriate the problem described above by use of this approach, a highly polished silicon wafer has been coated with carbon and then a sheet of polycrystalline silicon formed directly on top of the carbon layer. While this approach enabled easy separation of the sheet from the carbon layer when the carbon layer was essentially free from pinholes, an essentially pinhole-free carbon layer could be obtained only when the carbon layer was so thick that it was difficult to obtain the necessary flatness in the upper surface thereof and the process was economically unattractive due to the increased power requirements for heating the wafer/carbon unit during the growth step. Furthermore, it was difficult to remove the carbon adhering to the wafer after sheet separation, possibly due to the formation of silicon carbide compounds. In brief, this approach was not suitable for mass production techniques.
Accordingly, it is an object of the present invention to provide a method of obtaining a sheet of polycrystalline silicon which enables easy separation of the sheet from the substrate wafer.
Another object is to provide such method in which the substrate, or at least a major portion thereof, is reusable.
A further object is to provide such a method which is economical and adapted to mass production techniques.
A final object is to provide a workpiece which is useful in the practice of such a method and enables easy separation of the sheet from the substrate wafer.